We have a case of the utah doctor accused of murdering his wife. There's a new twist. The doctor's mistress has admitted to the affair in court. Saying he moved her into the family home just weeks after his wife's death. Linsey davis is here with the latest. Reporter: Jesse willis is her name. And she talks about her relationship with martin MacNEILL ON THE WITNESS STAND. Saying, it was just for fun. All eyes in the courtroom wednesday were on 24 woman, gypsy willis. You can see here, as she left the witness stand smiling, her eyes were glued on martin MacNEILL, THE MAN ABOUT TO STAND Trial for the murder of his wife, michelle. Prosecutors say the year-long affair with willis is the motive. The motive will sit well with the jurors. It gives them a hook to justify conviction. Reporter: In court, willis testified the two met online in 2005 and started a sexual relationship one year later. She also said he provided her with a credit card to help her get through nursing school. She testified she never thought MacNEILL WOULD LEAVE HIS WIFE For her. But just weeks after she died, he moved willis in the house as the new nanny. In court, she was referred to as the nanny with benefits. Not long after her testimony, the judge ordered the doctor to stand trial for first-degree murder. calling MacNeill's actions evidence of a guilty mind. MacNEILL WHO MADE THIS 911 CALL, Says he's innocent. That he tried to save his wife. He has done some got things in his life. Reporter: But in a search warrant, investigators said the doctor intentionally overdosed his wifeer she had plastic surgery. Surgery he insisted she have. The state will need to show that there's some conduct on behalf of the defendant that links him to the murder. Reporter: His daughters and other family members have consistently been a presence in the court, often clutching pictures of their mother, as they glared at their father in the courtroom. It's a whole web of lies. This was very well planned out. He almost got away with it. Let's hope that the jury convicts my father. Reporter: After the death of HIS WIFE, MacNEILL AND WILLIS Were charged with identity fraud and served prison. MacNEILL IS EXPECTED TO ENTER Pleas for the murder charges later this month. The trial likely won't start until next year. We have a long way to go. Thanks very much.

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